(1) Field of the Invention
This invention relates to a method and an apparatus for continuously forming embossed sheets. The embossed sheets that are formed according to the present invention can be used as core materials for producing hollow laminated plastic boards, light-weight plates, and the like.
(2) Description of the Prior Art
The core materials used for conventional hollow laminated plastic boards are roughly divided into two types. One is a flute type (corrugated type) and the other is an embossed type. Besides the above types, several modified types of them are also proposed.
For example, a corrugated sheet of the flute type is disclosed in British Pat. No. 1,020,889. Meanwhile, the disclosure on the embossed type is exemplified by U.S. Pat. No. 3,142,599.
These hollow laminated boards have various advantageous properties; however, there remain some defects in view of the following points.
That is, even though the hollow laminated boards of the flute type are suitable for making boxes and containers, the mechanical strength of the boards of this type varies with the directions of the boards. This directional property is disadvantageous in some uses. For example, secondary processing such as butt welding and thermoforming is not always suitable for boards of this type owing to the directional property or because they are easily crushed flat.
Meanwhile, the directional property is seldom observed in the core materials of the embossed type due to its structural features. The use of this material is, however, limited because the configurations of the top and bottom faces are different. When a liner sheet is applied to the core material, the obtained laminated board is liable to warp, which is also inconvenient in use.
As one of other core materials which are free from the above defects, there is an embossed sheet (cuspated sheet) disclosed in U.S. Pat. No. 3,963,813. This embossed plastic sheet has little directional property and the configurations of both faces are symmetrical. And the sheet is suitable as a core material for hollow plastic boards because when both sides of the sheet are simultaneously applied with liner sheets, the obtained laminated product is free from warping. However, the apparatus for producing the embossed sheet is complicated in structure, resulting in increased production cost.